Parahydrogen-induced polarization (PHIP) hyperpolarized MR receptor imaging in vivo: A pilot study of ¹³C imaging of atheroma in mice

MR techniques using hyperpolarized ¹³C have successfully produced examples of angiography and intermediary metabolic imaging, but, to date, no receptor imaging has been attempted. The goal of this study was to synthesize and evaluate a novel hyperpolarizable molecule, 2,2,3,3-tetrafluoropropyl 1- ¹³C-propionate-d _2,3,3 (TFPP), for the detection of atheromatous plaques in vivo. TFPP binds to lipid bilayers and its use in hyperpolarized MR could prove to be a major step towards receptor imaging. The precursor, 2,2,3,3-tetrafluoropropyl 1- ¹³C-acrylate-d _2,3,3 (TFPA), binds to 1,2-dimyristoylphosphatidylcholine lipid bilayers with a 1.6-ppm chemical shift in the ¹⁹F MR spectrum. This molecule was designed to be hyperpolarized through the addition of parahydrogen to the ¹³C-acrylate moiety by parahydrogen-induced polarization. TFPA was hyperpolarized to TFPP to an extent similar to that of the hydroxyethylacrylate to hydroxyethylpropionate transition: 17±4% for TFPP versus 20% for hydroxyethylpropionate; T ₁ relaxation times (45±2s versus 55±2s) were comparable and the hyperpolarized properties of TFPP were characterized. Hydroxyethylacrylate, like TFPA, has a chemical structure with an acrylate moiety, but does not contain the lipid-binding tetrafluoropropyl functional group. Hyperpolarized TFPP binds to the lipid bilayer, appearing as a second, chemically shifted ¹³C hyperpolarized MR signal with a further reduction in the longitudinal relaxation time (T ₁ = 21±1s). In aortas harvested from low-density lipoprotein receptor knock-out mice fed with a high-fat diet for 9months, and in which atheroma is deposited in the aorta and heart, TFPP showed greater binding to lipid on the intimal surface than in control mice fed a normal diet. When TFPP was hyperpolarized and administered in vivo to atheromatous mice in a pilot study, increased binding was observed on the endocardial surface of the intact heart compared with normally fed controls. Hyperpolarized TFPP has bio-sensing specificity for lipid, coupled with a 42 000-fold sensitivity gain in the MR signal at 4.7T. Binding of TFPP with lipids results in the formation of a characteristic second peak in MRS. TFPP therefore has the potential to act as an in vivo molecular probe for atheromatous plaque imaging and may serve as a model of receptor-targeted bio-imaging with enhanced MR sensitivity.